Present internal combustion engines are generally equipped with internal combustion engines that have relatively high power reserves. This means that the engines run most of their operating time with relatively low torque or, expressed differently, with relatively low effective average pressure. The relative consumption (i.e., the consumption normalized to engine torque), however, generally has its global optimum at a relatively high effective average pressure, and then exhibits only a relatively limited dependence on engine torque over a relatively broad range of average engine speed.
It is known that low-speed driving in a higher gear generally leads to a reduction in fuel consumption. The reason for this is that at lower engine speed, but constant engine power, the delivered torque and thus the relative effective average pressure is generally greater, which shifts the operating point into more favorable consumption characteristic regions (cf., for example, Kraftfahrtechnisches Taschenbuch/Bosch, 21.sup.st Edition, 1991, pages 322-323). The consumption can likewise generally be favorably influenced in automatic transmissions by selecting the switching points at low speeds.
With continuous transmissions (so-called CVT transmissions, in English "Continuous Velocity Transmission") or finely graded transmissions, these fundamental ideas can be used more effectively. In fact, the possibility of (quasi) continuous adjustment of transmission permits selection of the operating point, so that the consumption is minimal for the corresponding engine power (cf., for example, Karftftechnisches Taschenbuch, loc. cit., pages 550-551). In practice, however, there are often boundary conditions that restrict the range of possible transmission ratios and therefore do not permit, strictly speaking, achievement of the theoretically possible consumption minimum at a corresponding power. Consumption optimization then consists of selecting the operating point within the range of possible transmission ratios, so that the practically possible consumption minimum is achieved. One such boundary condition, for example, can be a transmission ratio limit stipulated by the transmission by design, say, when an "overdrive transmission" (i.e., transmission ratio of smaller than one), often not present in a transmission, would be necessary at smaller delivered power to achieve the theoretical consumption minimum. Another such boundary condition can lie in the fact that a certain acceleration excess can be desired, even in a continuous (or finely graded) transmission. This is understood to mean the distance of the torque at the operating point from the maximum torque.
Another proposal to shift the operating point as close as possible to the global consumption optimum (in a hybrid vehicle here) is known from DE 43 44 053 A1. The concept is not based on increasing the torque by reducing the speed at constant power, but by increasing the power demanded by the internal combustion engine. The excess power not required to drive the vehicle is converted to electrical power by a generator driven by the internal combustion engine and stored in an electrochemical energy accumulator. The stored energy is reused during electric drive of the vehicle.
It is known from DE 195 05 431 A1 that the total efficiency in a hybrid vehicle can be optimized, this being calculated by a product of internal combustion engine, generator and battery efficiency. It is mentioned in the document that a speed adjustment by means of a CVT transmission is also possible as so-called "efficiency-influencing parameter".